Inhibiting coke formation by coating gas turbine elements with silica

ABSTRACT

A method is taught for protecting hydrocarbon contacting surfaces of a gas turbine engine from carbon deposition by the application of a coating of silica.

The invention was made under a U.S. Government contract and theGovernment has rights herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods for preventing the deposition ofcarbon, or coke, on fuel wetted surfaces located in high temperaturezones of gas turbine engines. Coke deposition is an undesirable sideeffect caused by the catalytic-thermal degradation of hydrocarbon fuelsduring their consumption in gas turbine engines. Such deposition leadsto performance loss, reduced heat transfer efficiencies, increasedpressure drops, costly decoking procedures, and increased rates ofmaterial corrosion and erosion. The metals most prone to catalyze cokedeposition are those metals commonly found in the alloys utilized incomponents exposed to high temperature, fuel wetted environments of gasturbine engines, typically found in jet engines in the combustor andafterburner fuel delivery systems.

2. Description of the Prior Art

Carburization, or the formation of coke deposits, has been notedparticularly in high temperature environments where carbon containingfluids come in contact with metals or metal alloys. Exemplary of suchenvironments are high temperature reactors, such as refinery crackers,thermal crackers, distillation units for petroleum feedstock, and gasturbine components. Conventional methods used to reduce coke formationand carburization in steam cracking operations involve the steampretreatment of the surface to promote formation of a protective oxideskin. The surface may then be further protected by the deposition of ahigh temperature, stable, non-volatile metal oxide on the pre-oxidizedsubstrate surface by thermal decomposition from the vapor phase of avolatile compound of the metal.

While the chemical vapor deposition of an alkoxysilane has beendemonstrated to reduce the rate of coke formation in the pyrolysissection of an ethylene steam cracker by formation of an amorphous silicafilm on the internal surfaces of high alloy steel tubing at 700°to 800°C., no one to date has solved the problem of coke deposition on fuelcontacting hardware in gas turbine engines.

SUMMARY OF THE INVENTION

The present invention relates to a means for reducing coke formation onfuel contacting components of gas turbines, such as in the combustor andafterburner of a jet engine. A thermally resistant barrier is applied toprevent contact of the fuel with catalytic agents such as iron, nickel,and chromium, contained in the base metals from which fuel contactingcomponents are fashioned. Specifically, the fuel contacting componentsare coated with a thin, high temperature resistant layer of silica,which reduces the rate and severity of coke deposition on the surfaces.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Coke deposition has been found to be an undesirable side effect causedby the thermally accelerated degradation of hydrocarbon fuels duringtheir use for power generation in gas turbine engines. It is aparticular goal of the present invention to reduce the deposition ofcarbon on fuel contacting components of gas turbine engines such as fuelnozzles, fuel lines, and augmentor spray manifolds, and such other areasas lubrication systems and breather tubes.

It is known that hydrocarbon fuels may degrade either under hightemperature conditions, i.e. thermally, or under lower temperatureconditions in the presence of a catalytic material. One approach to theproblem in the past has been to regulate the quality of the fuelconsumed, so as to limit degradation thereof. However, as engines arerequired to run faster and hotter to achieve greater output, the abilityof present day hydrocarbon fuels to provide the required performancewithout coking is lessened.

Further, since many of the metals required for the construction ofhigher temperature gas turbine engines are catalytic to the degradationof hydrocarbon fuels, coke formation has become of greater concern.Accordingly, a method has been sought to increase the temperature atwhich engines may operate without degradation of the fuel and depositionof coke. It has now been found that this may be achieved by theapplication of a suitable coating to fuel contacting surfaces to act asa barrier between the hydrocarbon and the catalytic elements in thesurfaces. Certain high temperature resistant materials either do notparticipate in the mechanism of catalytic-thermal deposition of coke, orparticipate to a much lesser degree than such metals as iron, nickel,chromium, or their alloys. It has also been noted in the course of ourinvestigations that certain materials actually enhance the degradationof carbon containing fuels. These same materials, when exposed toelevated temperatures, cause any gums and/or varnishes which do form tocompletely burn away. A suitable coating has been found to comprise athin layer of silica, SiO₂, which may be applied by a number of methods.For example, the silica may be applied by chemical vapor deposition ofan alkoxysilane, by sputtering, or by deposition from a solution oftetramethylorthosilicate. Care must be taken in the coating to assure acomplete coverage of the substrate, a uniform thickness of silica, andpurity of the coating as applied. Choice of the coating technique may bedependent upon such factors as the composition of the substrate, and thedifficulty of application to all fuel contacting surfaces of the elementbeing protected. The silica coating may be applied in a thickness offrom about 0.00005 inches to about 0.001 inches, preferably from about0.0001 inches to about 0.0005 inches. Such a coating is stable inhydrocarbon fuels, and is thermally stable at temperatures from aboutminus 200° F. to about 2500°. Sub-coats or bond coats may be applied ifnecessary to achieve an adherent bonding to the substrate. Conventionalcleaning and preparatory steps should be taken prior to deposition ofthe coating to enhance adherence.

A number of primary factors were identified which relate to thedeposition of hydrocarbons in gas turbines. These include fuelcomposition, temperature, time, the availability of oxygen, and thepresence of catalytic materials in the surface of the fuel handlingcomponents. For an operating gas turbine, each of these factors has analmost infinite number of possible values, with the exception of thecomposition of the fuel contacting elements of the gas turbine engineitself. Accordingly, the present invention is directed to control of thesurface composition of the fuel handling components of the gas turbineengine, and specifically to the provision of silica coatings thereuponto reduce the deposition of carbon, or coking.

Alloys used in hydrocarbon fuel burning engines commonly contain metalswhich catalyze coke deposition, such as iron, nickel, and chromium.Thermal degradation occurs as a matter of course, and there are periodsduring the operation of turbine engines when fuel flow is very low, oras in the case of military engine augmentor plumbing, i.e. fuel feedtubes and spray manifolds, there is no fuel flow at all. During suchperiods, the temperature of the residual fuel left in the plumbing canrise, causing increased coke deposition from accelerated fueldegradation reactions and thermal cracking. The contributions of variousmetallic hardware surfaces to coke deposition were evaluated with a goalof determining the best method for reducing the formation and adherenceof coke. It has been learned that coking may be reduced by applicationof a surface layer of an anti-coking material to the surfaces of thefuel handling components of a gas turbine engine. Such anti-cokingmaterials may be of a nature to either reduce or inhibit the tendency ofcoke to adhere to the surface, or, conversely, to enhance the catalysisof the surface and increase the reactivity such that any gums andvarnishes which tend to form are caused to react further, breaking themdown to gaseous products which are eliminated.

It is also to be noted that the presence of certain particulate metallicoxide materials, such as alumina, ceria, cupric oxide, in the cokeinhibiting layer, may add to the surface catalysis. Such particulatesshould range in size from about 45 microns to about 150 microns, andpreferably from about 45 microns to about 75 microns in diameter, andshould comprise from about 10 percent to about 45 percent of the volumeof the coating layer.

Surfaces which may be coated for prevention of coking include fuellines, fuel nozzles, augmentor spray manifolds and other hydrocarboncontacting surfaces of gas turbines, such as lubrication systems andbreather tubes. Such surfaces may comprise such materials as titaniumand titanium alloys, aluminum, stainless steels, and nickel base alloyssuch as Inconel and Waspaloy. In addition, the present invention may besuitable for prevention of coking on other surfaces, such as copper,zirconium, tantalum, chromium, cobalt, and iron, for example. While theexamples which follow relate to coating components fashioned of Waspaloyor Inconel alloys, it is to be understood that the present invention isnot to be limited thereto.

To evaluate the effectiveness of experimental coatings in reducing thetendency of jet fuel to form coke deposits on a metal substrate,Waspaloy samples were utilized under conditions simulating theoperational conditions to be anticipated in a high performance militaryaircraft engine. In a typical military flight scenario, fuel is heatedas it travels through the fuel plumbing on its way to the combustorand/or augmentor of the engine to be burned. Generally, the fuel flowrate is sufficiently high to limit the effect of those factors whichrelate to coking. However, during flight, when the augmentor is shutoff, spray manifold temperatures in the afterburner section riseconsiderably, going from about 350° F. to about 1000° F. or higher insome areas. Fuel left in the spray manifold in these areas boils, andwith no place to flow, degrades rapidly to form insoluble, sticky,gum-like varnishes, which after a number of cycles results in formationof coke deposits. A similar scenario occurs in the engine combustor fuelnozzles at engine shutdown. However, since the augmentor is cycled onand off much more frequently than the engine is, it is to be expectedthat the augmentor fuel plumbing would have a higher coking rate thanthe combustor fuel nozzles. Accordingly, the conditions encountered atthe spray manifold of the augmentor section were selected as beingrepresentative of conditions which result in coke deposition.

EXAMPLE 1

Coatings of silica, alumina, and tungsten disulfide were initiallyevaluated for effectiveness against carbon deposition in the liquidphase, i.e. in flowing fuel with no boiling. Silica coatings wereapplied by dipping in a solution containing 41.3%tetramethylorthosilicate (TMOS), 38.9% methanol, and 19.8% distilledwater. The specimen surface was first pre-oxidized at 1000° F. The dipwas followed by air drying, and repeated four times, followed by firingat 1000° F. Sol gel alumina coatings were applied in a manner similar tothe TMOS silica, but in two sets of four dips each with firings at 1112°F. in vacuum (10⁻⁵ torr) for 5 hours between dip sets. The tungstendisulfide coatings were applied through an air blast gun at 120 psi,with the gun positioned 10 to 12 inches from the surface. The reductionsin coke deposition achieved are set forth in Table I. These are averagesof duplicate results as compared to uncoated specimens, subjected toidentical conditions.

                  TABLE 1                                                         ______________________________________                                        DEPOSIT REDUCTION                                                             Coating Type    Reduction                                                     ______________________________________                                        None            --                                                            Silica          28%                                                           Alumina         18%                                                           Tungsten disulfide                                                                             5%                                                           ______________________________________                                    

EXAMPLE 2

A second test was conducted using the coked test specimens from thefirst experiment. In this experiment the ability of each coating torapidly catalyze the gasification of the deposited coke was evaluated.The coked specimens from Example 1, along with the uncoated blankspecimens, were placed in a furnace at 1050° F. for two hours. Theoxygen content of the atmosphere in the furnace was lowered withnitrogen to simulate the reduced oxygen environment inside a spraymanifold. The results are set forth in Table II. The uncoated specimensshowed no observable reduction in coke deposits. The tungsten disulfidecoating, however, suffered some coating loss, apparently due toexceeding its upper temperature capability in air. No coating loss wasnoted for the other coatings.

                  TABLE II                                                        ______________________________________                                        DEPOSIT LOSS                                                                  Coating Type    Reduction                                                     ______________________________________                                        None            --                                                            Silica          100%                                                          Alumina          90%                                                          Tungsten disulfide                                                                            100%                                                          ______________________________________                                    

EXAMPLE 3

A Jet Fuel Thermal Oxidation Tester (JFTOT) was used to evaluate varioustubes of Waspaloy and Inconel 625 under test conditions chosen tosimulate, as closely as possible, the conditions at spray manifoldlocations of a military jet engine after augmentor cancellation. Theseconditions were as follows:

    ______________________________________                                        Temperature:        575° F.                                            Fuel flow:          2 ml/minute                                               Pressure:           400 psi, in air                                           Flow mode:          Recirculate                                               Time:               8 hours                                                   ______________________________________                                    

Three sets of Waspaloy oxidation test tubes were fabricated havingsurface finishes of 16, 32, and 63 microinch (u"), respectively, as wellas one set of Inconel 625 tubes with a 32 microinch finish. Coatings ofsilica (SiO₂), gold (Au), and tungsten disulfide (WS₂) were applied tosections of the test tubes for evaluation.

The silica coatings were applied by two different techniques. The firstwas by a conventional sputtering method, and the second by dipping thetest tubes in a solution containing 25 grams of tetramethylorthosilicate(TMOS), 30 ml methanol, and 12 ml distilled water. The dip was followedby air drying, and then repeated four times. The coating was then heatedat 150° F. to 200° F. for 30 minutes.

The tungsten disulfide coating was applied through an air blast gun at120 psi, with the gun positioned 10 to 12 inches from the surface.Tungsten disulfide does not adhere to itself, and therefore only amonolayer thickness of about 0.00002 inches or less was obtained.Alternatively, a thin layer of tungsten disulfide may be applied bymechanically brushing on a pre-oxidized Waspaloy surface, withsubsequent heating to about 800° F. to produce an adherent layer.

The gold coating was applied to the tube surface from a conventionalgold plating bath, with care taken to insure complete coverage with noareas uncoated.

The fuel used for the testing was JP-4 aviation fuel, taken from asingle source maintained at constant temperature.

Initially, the Jet Fuel Thermal Oxidation Tester was set up with aquartz heater tube housing to allow the use of a Remote Probe TubeDeposit Rater, which monitored, via strip chart recorder, the decreasein reflected light from the tube surface as carbon deposits are formed.The probe was aligned at the heater tube hot spot as determined by athermocouple inserted in the longitudinal hole of the heater tube. TheRater provided an estimation of time to the beginning of carbondeposition, and the rate of formation of the deposit. Since this deviceis an optical sensor, and coke deposit density is variable, noquantitative weight conversions were possible for this test. However,calculations of the percentage change in the rate of deposit were madeby comparing the slopes of the appropriate uncoated blank test tubes tothose of the coated tubes.

Tests were conducted upon Waspaloy 16 microinch, 32 microinch, and 63microinch uncoated tubes, and the coated tubes. Test results are shownin Table III, below.

                  TABLE III                                                       ______________________________________                                        OPTICAL TEST RESULTS                                                          Tube type/coating  Slope   Reduction                                          ______________________________________                                        Waspaloy, 16 u"/none                                                                             0.1203  --                                                 Waspaloy, 32 u"/none                                                                             0.1111  --                                                 Waspaloy, 63 u"/none                                                                             0.1008  --                                                 Inconel 625, 32 u"/none                                                                          0.1918  --                                                 Waspaloy, 32 u"/SiO.sub.2 sput.                                                                  0.1350  21.5% inc                                          Inconel, 32 u"/SiO.sub.2 sput.                                                                   0.1340  30.1%                                              Waspaloy, 32 u"/WS.sub.2                                                                         0.0675  39.0%                                              Waspaloy, 63 u"/Au 0.0691  37.6%                                              ______________________________________                                    

The deposit rate reduction for the Inconel 625, 32 microinch surfacefinish, with a sputtered silica coating was 30%, but for a Waspaloy, 32microinch surface finish with the same coating, an increase in depositrate of 21.5% was obtained. It was also noted that the deposit rates(slopes) for both were nearly identical, indicating that duringsputtering catalytic impurities were introduced to both coatings,causing deposit to form at the same rate on both tubes. Accordingly,these results were not considered indicative of silica coatingperformance.

The tungsten disulfide coated Waspaloy, 32 microinch surface finish, andthe gold plated Waspaloy, 63 microinch surface finish tubes provideddeposition rate reductions of 39% and 37.6%, respectively. The goldsurface was considered to present questions of long term durability,however, as well as expense, and was dropped from further consideration.

While the light reflective mechanism of the above tests provides a goodmeans for the determination of time to on-set of coke deposition, anddeposition rate, it is also desirable to know what the long termdeposition rate is. Therefore, using the same Jet Fuel Thermal OxidationTester conditions, but with the Remote Probe Tube Deposit Rater removed,a gravimetric analysis was made of a series of tests at 4, 8, 16, and 24hours. The tube weight gains were evaluated for both amount andreproducibility.

In the first series of the gravimetric tests, a Hot Liquid ProcessSimulator, an expanded capacity Oxidation Tester, was utilized. UncoatedWaspaloy, 32 microinch surface finish tubes were run to establish a newcoke deposition baseline. As shown in Table IV, the tungsten disulfidecoated Waspaloy, 32 microinch surface finish tube produced an averageweight reduction of 87%.

During a subsequent test run using a TMOS silica coated tube, the HotLiquid Process Simulator pump failed, necessitating resumption oftesting on the original Jet Fuel Thermal Oxidation Tester. After runningan uncoated Waspaloy, 32 microinch surface finish tube to establish anew baseline, additional coated tubes were run on TMOS silic coatedWaspaloy 16 microinch, 32 microinch, and 63 microinch finish surfacetubes. Results are set forth in Table IV. T1 TABLE IV-GRAVIMETRIC TESTRESULTS? -Tube type/coating? Deposit? Reduction? -Waspaloy, 32u"/uncoated 5.330 mg --? -Waspaloy, 32 u"/WS₂ 0.680 mg 87.2% -Waspaloy,32 u"/uncoated 0.950 mg --? -Waspaloy, 16 u"/TMOS SiO₂ 0.775 mg 18.4%-Waspaloy, 32 u"/TMOS SiO₂ 0.705 mg 25.8% -Waspaloy, 63 u"/TMOS SiO₂1.070 mg 12.6% inc? -

The TMOS silica coated Waspaloy yielded significant reductions in cokedeposition on both the 16 microinch and 32 microinch surface finishes.The TMOS silica coated Waspaloy, with the 63 microinch surface finishshowed no reduction in deposit.

EXAMPLE 4

Considering these results, the same coatings were tested to determinetheir ability to promote the gasification of coke deposits underconditions similar to those thought to exist in an operating enginebetween augmentation cycles, i.e. after shutdown of the afterburners. Ifsuch were the case, and the specific coating did not permit a greatercoke deposition rate than did the Waspaloy, then the initial depositswhich did form might be removed during higher temperature periods whenthe augmentor was shut down. If the removal rate were great enough, thendeposits would be removed almost as they formed.

To do this, a furnace was set up with a nitrogen purge to reduce the aircontent to approximate that existing in the spray ring area afteraugmentor cancellation. Blank and coated Waspaloy Jet Fuel ThermalOxidation Tester tubes which had been previously coked as in Example 3were placed in the furnace and heated to 1050° F. for two hours. Weightchanges were recorded, but apparently substrate oxidation weight gainsoffset some weight loss from coke gasification, as apparent fromexamination of the tubes under magnification. The uncoated (blank) tubeshad lost some, but very little, deposit. The coated tubes ranked as setforth in Table V with respect to the reduction of coke deposit.

                  TABLE V                                                         ______________________________________                                        DEPOSIT LOSS                                                                  Coating type     Reduction                                                    ______________________________________                                        Tungsten Disulfide                                                                             30%                                                          Sol gel alumina  90%                                                          TMOS silica      100%                                                         ______________________________________                                    

These results are indicative that even if small coke deposits occurduring augmentation cycling, those deposits may be completely gasifiedduring the "off" cycle of the augmentor, if the augmentor surface isprotected by a coating of silica. These results also indicate thatprotective surface coatings may be applied to fuel contacting elementsto inhibit carbon degradation and coking. It may also be seen from theresults shown in Examples 1 through 3 that as the surface chemicalreactivity (i.e. catalytic nature), roughness, and/or porosity, of thesubstrate increases, one may anticipate increased carbon deposition.

It is to be understood that the above description of the presentinvention is subject to considerable modification, change, andadaptation by those skilled in the art to which it pertains, and thatsuch modifications, changes, and adaptations are to be considered withinthe scope of the present invention, which is set forth by the appendedclaims.

What is claimed is:
 1. A method for the inhibition of coke formation ona gas turbine engine element in contact with a liquid hydrocarbon fuelat a temperature range of from about 350° F. to about 1000° F., saidelement comprising a metal selected from the group consisting oftitanium, titanium alloys, aluminum, stainless steel, and nickel basesuperalloys, said method comprising preoxidizing the surface of saidelement, and applying an adherent inert layer of silica to the surfaceof said element by deposition from a solution oftetramethylorthosilicate, wherein said layer of silica is from about0.00005 inches to about 0.001 inches in thickness and further comprisesmetallic oxide particles selected from the group consisting of alumina,ceria, and cupric oxide.
 2. The method of claim 1, wherein saidparticles are from about 45 microns to about 150 microns in size.
 3. Themethod of claim 2, wherein said particles comprise from about 10 percentto about 45 percent of the volume of said layer.